476 



Discussion 



endoplasm where microsomes containing cytochrome A-, would be expected. The 

 absorption peaks correspond to those of reduced cytochromes a^, b, and c in the 

 mitochondria and to 65 or the 'cytoplasmic pigment' in the microsomes. The peak 

 positions are appropriate to the reduced forms of these substances in an 'absolute' 

 rather than a difference spectrum. These data for the single cell teased directly from 

 the tissue indicate a considerable concentration of the reduced 'cytoplasmic pigment' 

 without the addition of DPNH or dithionite, as was necessary for recording absorbency 

 changes of the cells in suspension (Figs. 2 and 3). This result agrees with those of 

 MacMunn and Keilin for liver tissue but clearly distinguishes the location of the 

 cytochromes O3, b, c and b^ spectra. 



Observations of the cytoplasmic pigment in the transition from aerobiosis to 

 anaerobiosis in cell suspensions support the observations of Fig. 1 : no change is 



Fig. 5. Microspectrophotometric recording of the 

 spectra of a single, teased and centrifuged, anaerobic 

 liver cell. B, base line; 1, mitochondria; 2, endo- 

 plasm. (BT-4.) Reprinted from Exptl. Cell Research 

 20, 43, 1960. 



-X 



observed. However, significant increases of reduction are observed as a function of 

 time after beginning the experiment, which might well be due to variations in the 

 level of cytoplasmic DPNH. 



Nomenclature of accessory and cytoplasmic pigments — These studies and those 

 reported by Chaix (p. 225) and by Bonner (p. 479) lead us to emphasize the use of 

 the term accessory pigment (Chance and Williams, Advanc. Enzymol. 17, 65, 1955) 

 in contrast to respiratory pigment. An acceptable preliminary test for the latter is the 

 existence of a steady state of the pigment in respiratory activity as observed at room 

 temperatures or as 'trapped' at liquid nitrogen temperatures. Another test is based on 

 observations of the increasing reduction of the pigment that closely follows the time- 

 course of the termination of respiration as oxygen is exhausted in an aerobic-anaerobic 

 transition. An accessory pigment may not change its oxidation-reduction level at 

 all during this transition or may change so slowly that its kinetics violate one of the 

 two theorems mentioned elsewhere in this symposium (see p. 607). A critical test is 

 afforded by a comparison of the kinetics of its oxidation and reduction with the rate 

 of electron transfer through the system. There should be a close relationship for the 

 respiratory pigment and a remote one for the accessory pigment. This does not, 

 however, eliminate the participation of the accessory pigment from slow phases of 

 biological oxidation. If the localization of the accessory pigment is known, then the 

 term may be appropriately modified, as in this case, to cytoplasmic pigment; if the 

 pigment is localized elsewhere, such terms as endochrome, mitochrome, nucleochrome, 

 etc., might ultimately become more useful and more sharply define the general term 

 cytochrome. 



Possible Functions of the Cytochromes of the Enchplosniic Reticulum of 



Animal Cells 



Dickens: My remarks about tlie paper by Stritlmatter (p. 461) concern the possible 

 significance of the reduced di- and triphosphopyridine nucleotide (DPNH and TPNH) 

 cytochrome reductase activity of microsomes. Whereas in the isolated microsome 

 fraction of liver and other tissues cytochrome 65 readily interacts with DPNH and 



